Returns Manager does not integrate with other return systems "out of the box" (ex: 3PL, Shipwire, outside APIs). At this time, open API access is not available for Returns Manager.
Integrating Returns Manager with another system, such as these, would require custom development.
Bold Returns Manager is no longer available to install on the Shopify App Store. If you already have Bold Returns Manager installed, you will be able to continue using it as it is grandfathered on your store.